Experiments in Analytical Chemistry -Spectrophotometric determination of Mn(II) and Cr(III)

Electromagnetic waves

Energy units and conversion

Beer’s Law  Transmittance (T) vs Absorbance (A)  Beer’s law  : molar absorptivity, cm -1 M -1 b : path length, cm c : molarity, M

Measuring UV-visible spectrum

Spectrophotometer Single-beam vs double-beam Double-beam spectrophotometer

Reflection grating n = a – b = d (sin  + sin  ) n = 1, 2, 3,… Resolution of grating =  nN Dispersion of grating =  n  dcos 

Detectors Photomultiplier tube (PMT) Diode array detector

Materials and chemicals for Mn and Cr determination Reagent grade K 2 Cr 2 O 7, KMnO 4 K 2 S 2 O 8, KIO 4, AgNO 3 Conc. H 2 SO 4 3% H 2 O L volumetric flasks mL volumetric flasks, mL volumetric flasks, mL volumetric flasks Solutions Standard solutions of Mn(II) and Cr(III) Cr 3+ solution Prepare a standard M of Cr(III) in 1 M sulfuric acid. Weigh to the nearest milligram 1.48 g of reagent grade potassium dichromate into a clean 500 mL flask. To this add 100 mL of distilled water, 5 mL of conc. Sulfuric acid and 25 mL of 3% hydrogen peroxide. After the reduction of dichromate to chromium(III) has occurred, heat the solution to boiling for 15 to 20 min to decompose the excess H 2 O 2. Cool the solution and transfer it to a 1-L vol flask. Add distilled water along with 50 mL of conc H 2 SO 4.

Mn 2+ solution Prepare M Mn(II) in 1 M sulfuric acid. Pipette exactly mL of a standardized 0.02 M potassium permanganate solution into a clean 500 mL Erl. Flask. Add 50 mL of distilled water, 5 mL of conc sulfuric acid, and 10 mL of 3% hydrogen peroxide. After the reduction of permanganate appears to be complete heat the solution to boiling for about 15 to 20 min to decompose the excess hydrogen peroxide. Cool the solution and transfer it quantitatively to a 1-L vol flask. Add distilled water, plus 50 mL of conc sulfuric acid, to bring the volume to the calibration mark. Stopper the flask, mix the solution thoroughly, and calculate the exact concentration of Mn(II) and label the flask appropriately.

Procedure 1.To each flask add 5 mL of concentrated sulfuric acid and 1 mL of 02 M silver nitrate solution. 2.Dilute each solution with distilled water to a volume of approx 50.0 mL. 3.Then dissolve 5 g of reagent-grade potassium persulfate in each solution. 4.Heat the solutions to boiling, and add 0.5 g of reagent-grade KIO 4 to each solution. 5.Bring the solutions to boiling for a period of five min. 6.Cool each solution to room temperature and transfer it quantitatively to a clean 100-mL vol. flask and dilute the solutions with distilled water to the mark. 7.Obtain the complete absorption spectrum for each of the standard solutions as well as for the unknown solution in the wavelength region from 600 to 375 nm. 8.Follow instructions given for the instrument. 9.From the data obtained with pure permanganate and dichromate solutions, calculate an apparent molar absorptivity for each species at each desired wavelength. 10.Calculate the concentrations of chromium(III) and manganese(II) in the original sample solution.